1. Field of the Invention
The present invention relates to RF power amplifiers and methods of RF signal amplification. More particularly, the present invention relates to RF feed forward amplifiers and related methods of signal amplification.
2. Description of the Prior Art and Related Information
FIG. 1 illustrates a conventional feed forward amplifier design having a main amplifier path 1 and an error amplifier path 2 with the error path designed to substantially eliminate distortion introduced into the main amplifier path through the operation of the main amplifier. As the amplifier ages and as other circumstances affect the operation of the amplifier, the effectiveness of the error cancellation will typically be degraded. To compensate and to provide a dynamic correction of such effects, a pilot tone has been employed which is injected into the main amplifier path. The pilot tone is typically chosen so that it is readily distinguished from the input signal to be amplified. Since the pilot signal is only injected into the main amplifier path and not the error path, it should in principle be treated in the same manner as distortion introduced by the amplifier and should therefore be eliminated by the operation of the error path. If the pilot tone is detected at the output of the feed forward amplifier, its presence indicates a lack of perfect cancellation of distortion introduced into the main amplifier path. This detected pilot tone can then be used to control parameters in the main or error path to attempt to eliminate the pilot tone and therefore improve the compensation of the error path for errors introduced into the main signal path.
In many high power applications of feed forward amplifiers the main amplifier (and possibly error amplifier) comprises a number of individual amplifier stages or legs which are coupled together in various architectures. Two such common architectures integrating a number of smaller amplifier stages 3 into a larger amplifier are illustrated in FIGS. 2A and 2B. The use of a number of smaller amplifiers has a number of advantages such as reduced cost and improved heat dissipation. However, one disadvantage occurs due to the possibility of a single one of the amplifiers failing. In such case, it may not be easily detectable that an amplifier stage has failed. In particular, the compensation controls built into the amplifier may treat the slight reduction in power in either the main or error amplifier as distortion or some type of parameter change which requires compensation. This can then cause the control system to overdrive the amplifier causing distortion in the overall output and/or premature failure of the system. Also, with the loss of a single stage in one of the amplifiers being undetected the nature of a fluctuation in the quality of the output signal magnitude may be mostly attributed to other factors and necessary maintenance not performed. Therefore, it is important to provide a simple and effective way of checking whether or not a single amplifier stage in an amplifier network in a feed forward amplifier arrangement has failed.
Prior approaches to detecting such single amplifier stage failure have suffered from various deficiencies. For example, one approach monitors the power output to each amplifier stage looking for a drop in output indicating a potential failure. This approach has a disadvantage in that amplifier output may drop due to variation of the input signal for one reason or another. This may trigger a false amplifier failure indication of the system. Also, there is inherent cross-coupling in the power of the various stages and this can also interfere with accurate detection of failure of an individual amplifier stage in this method. Other approaches are also known for detecting single stage failure but they also suffer from reliability problems and other disadvantages such as complexity and cost.
Therefore, a problem presently exists in feed forward amplifier designs due to the lack of an effective and efficient way of detecting amplifier stage failure where the stage is in a large parallel architecture.
The present invention provides an efficient and effective system and method for detecting individual amplifier stage failure using pilot tones.
In a first aspect the present invention provides an RF power amplifier comprising an amplifier input for receiving an RF signal to be amplified, an amplifier output for outputting an amplified RF signal, and a multi-stage amplifier comprising a plurality of separate amplifier stages coupled between the amplifier input and amplifier output so as to cumulatively provide a desired amplifier gain to the input RF signal. A pilot tone generation circuit injects a pilot tone into the multi-stage amplifier. An amplifier control circuit is coupled to the plurality of separate amplifier stages and cycles the amplifier stages ON and OFF. A failure detection circuit is coupled to detect the pilot tone output from the multi-stage amplifier and detects individual amplifier stage failure based on the detected pilot tones.
In a preferred embodiment, the amplifier control circuit controls the ON/OFF operation of the amplifier stages such that each stage is ON during a unique time slot for each amplifier stage. The failure detection circuit is coupled to the amplifier control circuit and receives the unique time slots therefrom and the failure detection circuit detects individual amplifier stage failure by comparing the detected pilot tone to the individual time slots corresponding to each amplifier stage. The amplifier control circuit may be separately coupled to the amplifier stage power supply inputs via a plurality of switches and controls the opening and closing of the plurality of switches based on the unique time slot for each amplifier stage.
In a further aspect the present invention provides an RF power amplifier with amplifier stage failure detection which comprises an amplifier input for receiving an RF signal to be amplified, an amplifier output for outputting an amplified RF signal, a plurality of separate amplifier stages coupled between the amplifier input and amplifier output so as to cumulatively provide a desired amplifier gain to the input RF signal, and means for detecting amplifier stage failure by providing a pilot tone to the amplifier stages, duty cycling the stages based on a separate timing for each stage and detecting the pilot tone output from the stages based on the timing. The means for detecting amplifier stage failure preferably comprises amplifier control means for duty cycling the amplifier stages based on a separate timing for each stage and failure detect means for detecting the pilot tone output from the stages and detecting stage failure when a pilot tone is missing at a timing corresponding to a stage ON cycle. The amplifier control means preferably comprises a plurality of switches coupling the amplifier power supply to the amplifier stages, and an amplifier control circuit coupled to the switches and providing an ON/OFF control signal to the switches based on the timing. The amplifier control circuit may be separately coupled to each of the switches.
In a further aspect the present invention provides a feed forward amplifier comprising an RF input for receiving an RF signal and a main amplifier signal path coupled to the input comprising a main amplifier having a plurality of separate amplifier stages. A pilot tone generator generating a pilot tone is coupled into the main amplifier input. A failure detection monitoring circuit is coupled to the main amplifier output. An amplifier control circuit controls the ON operation of the separate amplifier stages based on separate time slots. The main amplifier signal path of the feed forward amplifier further comprises a first delay circuit coupled to the output of the main amplifier. The feed forward amplifier further comprises an error amplifier signal path coupled to the RF input and receiving a gain reduced output from the main amplifier. The error amplifier signal path comprises a second delay circuit coupled to the RF input, a combiner coupling the delay circuit output to the gain reduced output from the main amplifier so as to be out of phase therewith, and an error amplifier for amplifying the output of the combiner. The feed forward amplifier further comprises a second combiner combining the output from the error amplifier and the first delay circuit so as to be out of phase therewith, and an output coupled to the second combiner and providing an amplified RF output. The amplifier control circuit preferably comprises a plurality of switches coupled between the amplifier power supply and respective amplifier stages and a control circuit coupled to the switches and controlling the opening and closing of the switches. In particular, the control circuit provides a sequence of control signals in a time ordered sequence to the plurality of switches. The amplifier control circuit also provides the time ordered sequence to the failure detection circuit and the failure detection circuit provides a failure detect signal when a pilot tone is missing in a time slot corresponding to an amplifier stage. The sequence of control signals may be ON/OFF control signals.
In a further aspect the present invention provides a method for detecting amplifier stage failure in an RF amplifier having a main amplifier with a plurality of amplifier stages. The method comprises providing a pilot tone to the main amplifier, duty cycling the amplifier stages with a unique time slot for each amplifier stage, detecting the pilot tone output from the main amplifier, and providing an amplifier stage failure signal if a pilot tone is missing in a time slot corresponding to an amplifier stage. In a preferred embodiment the act of duty cycling the amplifier stages comprises providing an ON/OFF control signal to a plurality of switches respectively connected to each amplifier stage, and opening and closing the switches with a timing such that each stage is ON in a unique time slot. The act of duty cycling the amplifier stages may be performed periodically during amplifier operation, during amplifier start up, and/or during an amplifier test.
Further aspects of the invention will be appreciated from the following detailed description of the invention.